Final averages are typically approximately normally distributed with a mean of 78 and a standard deviation of 9. Your professor says that the top 7% of the class will receive an A; the next 17%, a B; the next 50%, a C; the next 16%, a D; and the bottom 10%, an F. (Give your answers correct to one decimal place.)
(a) What average must you exceed to obtain an A?

Question

Final averages are typically approximately normally distributed with a mean of 78 and a standard deviation of 9. Your professor says that the top 7% of the class will receive an A; the next 17%, a B; the next 50%, a C; the next 16%, a D; and the bottom 10%, an F. (Give your answers correct to one decimal place.) 
(a) What average must you exceed to obtain an A?

dumbcow · Answer

use z-scores and a standard normal probability table

z = x-mean/std dev

z = (x-78)/9

Look up 0.93 in the table and see which z-score it is associated with, plug that number in for "z" and solve for x

dumbcow · Answer

you should get

z = 1.475